Representation of a so-called volume capnogram is common in prior-art respirators to enable the caregivers to assess the breathing process, for example, in case of such a device used in intensive care.
FIG. 1 shows such a volume capnogram as an example.
To obtain such a volume capnogram, measured values for an indicator of an expired volume and measured values for an indicator of a carbon dioxide concentration—CO2 concentration for short—are recorded for the breathing gas of a test subject. For example, a quantity of air flow that can be measured with a flow sensor may be used as an indicator of an expired volume. For example, the actual CO2 concentration (FCO2) itself or a CO2 partial pressure (PCO2) may be used as the indicator of a CO2 concentration. A CO2 sensor may be used to measure the CO2 concentration. The measured values are usually recorded at preset or presettable, normally equidistant points in time, so that a plurality of value pairs are obtained.
Two measured values each, recorded at the same time or at least essentially at the same time, form a value pair. The recorded measured values are measured values for an indicator of a carbon dioxide concentration, hereinafter called concentration measured values for short, and measured values for an indicator of the expired volume, hereinafter correspondingly called volume measured values for short. Each value pair correspondingly comprises a concentration measured value and a volume measured value. A linear curve is obtained by the graphic representation of the measured values recorded during an expiration process in a Cartesian system of coordinates, namely, when representing the concentration measured values over the corresponding volume measured values, and the entirety of the measured values thus represented forms the volume capnogram. The volume measured values are plotted on the abscissa and the concentration measured values on the ordinate.
The volume capnogram recognizably has three sections, which are called phase 1, phase 2 and phase 3, beginning from the left, in the scientific literature, and are designated by P1, P2 and P3 in FIG. 1.
At the beginning of expiration, breathing gas (gas), which has not participated in the gas exchange with the blood, reaches from the airways the respective sensor, e.g., the CO2 sensor. It correspondingly contains only a small percentage of or no CO2 (phase 1). The measurable CO2 concentration increases appreciably (phase 2) only when gas from the alveoli of the lungs reaches the CO2 sensor. The CO2 concentration reaches a plateau at the end of the rise phase, and the CO2 concentration normally continues to rise at least slightly even within the plateau (phase 3).
Two parameters are of special significance in the evaluation of the capnogram: On the one hand, the volume beginning from which the CO2 concentration rises and, on the other hand, the change in the CO2 concentration in the area of the plateau.
The volume beginning from which the CO2 concentration rises describes the expired quantity of gas, which has not reached the alveoli and could not therefore participate in the gas exchange with the blood. At least the gas content in the mouth, pharynx and upper part of the trachea of the test subject belongs to this quantity of gas. There is no gas exchange in this tidal volume area via the alveoli. The CO2 concentration will consequently correspond essentially to the CO2 concentration in the air inspired previously. This volume is called a serial dead space or, after subtracting the gas volume of the measuring device, also anatomic dead space. It is designated by the symbol Vds in the literature. The dead space or a change in the dead space during a therapy or during a longer-lasting observation of a test subject can be used as an indication of changes in the lungs or airways.
The change in the CO2 concentration in the area of the plateau is quite generally an indicator of the quality of the gas exchange in the lungs.
The method according to Fowler (Fowler W. S., Lung function studies II: The respiratory dead space, Am. J. Physiol., Vol. 154 (1948), pp. 405-416) has come commonly into use and is frequently used for the determination of these two parameters.
The measured value curve in FIG. 1 is also called FCO2 curve for the further explanation, because the measured value curve represents the carbon dioxide concentration (FCO2) during the expiration process, i.e., over the volume measured values. Fowler's method begins by drawing a straight line through the plateau of the FCO2 curve in phase 3 “with the naked eye.” A vertical line is subsequently positioned in the course of the FCO2 curve in phase 2. The position of the vertical line is selected to be such that an area to the left of the vertical line and an area to the right of the vertical line are equal or at least approximately equal. The area being considered here to the right of the vertical line is defined, on the one hand, by the line itself, then the FCO2 curve and finally the straight line drawn through the plateau. The area to the left of the vertical line is likewise defined by the line itself and the FCO2 curve as well as the abscissa of the system of coordinates (FCO2 zero line). FIG. 2 shows this fact graphically. The two areas to the left and right of the vertical line are designated by A1 and A2, respectively, in the diagram. Such an evaluation has been performed so far only by medical staff trained in this field based on an examination of the capnogram, and the position of the vertical line was essentially estimated in light of the equality of the areas adjoining on the right and left. An actual determination of the contents of the two areas adjoining on the right and left and hence an exact positioning of the vertical line to determine the serial dead space has been performed essentially for scientific publications and the like.
An automatic evaluation of a volume capnogram, called an expirogram there, has become known from DE 10 2004 039 194 A. Reference is made to this document to avoid repetitions that are unneeded here, e.g., in respect to the explanation of some technical terms such as “dead space” or generally in respect to the explanation of the physiological principles of gas exchange in the lungs.
The approach from DE 10 2004 039 194 A uses a certain type of function, which comes close to an ideal capnogram. Such an approach is not flexible enough to identify capnograms that considerably differ therefrom and are hence unusable without human interaction.